Mitt Romney is standing by his words from a private fundraiser that were caught on videotape. In the video, which was obtained by the liberal magazine Mother Jones, Romney says 47 percent of the country pay no income tax and are therefore predisposed to vote for President Obama.

How accurate is this number -- 47 percent -- that Romney cited?

"Roughly half of Americans pay no income tax," says Roberton Williams, senior fellow at the Tax Policy Center in Washington. That's down slightly from a few years ago, and some estimate it will fall below 40 percent by the end of the decade.

Of those people not paying income tax, about half of them are people with no taxable income -- low-income elderly and low-income working families.

But the other half, Williams says, are people who benefit from tax preferences, like the child credit or the earned income tax credit.

The idea that they are paying no taxes whatsoever is somewhat misleading, though. According to Williams, about two-thirds payroll taxes (because they work), while most of them will also pay state income taxes or sales taxes.

"We all benefit from the tax preferences in the system," Williams says, "and the people who benefit most are the wealthiest Americans. Mitt Romney himself benefitted hugely from the 15 percent preferential rate on capital gains."